Antiaromaticity versus Electron Affinity: Quantitative Insights into Metal–Ligand Interactions in Ni–Pentalene Complexes

Transition metal complexation has long served as a strategy for stabilizing antiaromatic hydrocarbons. However, the underlying factors that govern their coordination behavior are not yet fully understood. In this study, we synthesized and characterized a Ni(0) complex, [Ni(cod)(1)], featuring a benzothiophene-S,S-dioxide-fused pentalene ligand, and compared it to a structurally analogous complex, [Ni(cod)(2)], bearing a more antiaromatic but less electron-accepting pentalene. Through a combination of experimental and computational approaches, including single-crystal X-ray diffraction, X-ray absorption spectroscopy, variable-temperature NMR, and energy decomposition analysis, we found that the enhanced stability of [Ni(cod)(1)] is primarily associated with substantial π-back donation facilitated by its low-lying LUMO, rather than by the relief of antiaromatic destabilization. Notably, [Ni(cod)(1)] also exhibits pronounced fluxional behavior in solution, involving reversible migration of the Ni center between the two fused five-membered rings. DFT calculations suggest that this dynamic process is coupled with the shifting of C=C bonds within the pentalene framework. These findings illustrate that focusing on quantifiable electronic features provides a more nuanced and predictive understanding of antiaromatic coordination chemistry, while also revealing dynamic behaviors that may be intrinsic to antiaromatic frameworks.